feat(progressbar): Adding default and custom texts to bars

I've added the possibility to print the units of the bars as a percentage. To do so, it's only needed to add the attribute `auto-text="true"`.

It's also possible to specify any custom text with the object format in the 'text' property, overriding the previous behaviour:
```javascript
$scope.myObj = {
  value: 10,
  text: 'Ten'
}
```